The output voltage waveform of voltage inverters for vector control of AC electric motors by large power voltage inverters is not a sine wave but a PWM modulated rectangular wave. Consequently, there is the problem of waveform distortion due to higher harmonics.
To address this problem, various devices for digitally controlling sine wave tracking have been proposed. FIG. 5 is a diagram showing the system configuration of such a digital control device for sine wave tracking. This digital control device is configured by a compensator, a control object, and a feedback gain (h). The compensator is provided for controlling the tracking of the output y[i] of the control object to a reference waveform yr[i].
Ordinarily, an integrator as shown in FIG. 6 is often used for the compensator in FIG. 5. However, even when performing compensation with this integrator, sine-wave tracking control is not possible, and deviations occur. With the control device in FIG. 6, when determining the gain by deadbeat control for a second-order control object, and trying to track a sine wave, a result as shown in FIG. 7 is obtained. The width between the two curves in FIG. 7 shows the deviation between the sinusoidal reference waveform and the control result with the control device in FIG. 6. FIG. 8 shows a graph in which the sample points of the initial portion 11 in FIG. 7 have been enlarged. As becomes clear from FIG. 8, a deviation occurs with the control device of FIG. 6.
On the other hand, there are also configurations provided with a compensator due to repetitive control as shown in FIG. 9, based on the internal principle model. However, with a compensator using this repetitive control, a configuration becomes necessary whose order corresponds to one cycle. Therefore, in order to control a 50 Hz sine wave with a sample time of 100 μS (microseconds), a compensator of the 200th order becomes necessary. It should be noted that the “200” of the “200th order” is calculated by “( 1/50)/0.0001).” An explanation of repetitive control is given for example in “KISO DIGITAL SEIGYO (Basic Digital Control),” Corona Publishing, p. 108.